§ too ELECTRICAL ENGINEERING PRACTICE
a-bismuth-antimony thermo-couple (§ 122) connected in the circuit
.of a low resistance moving coil, permanent magnet instrument
('§ 101). This instrument combines independence of wave form
and frequency with the high sensitivity obtainable by using
a powerful permanent magnet, whilst avoiding- the mechanical
•' difficulties of the ordinary hot-wire instrument (see also § 118).
100. Moving Iron Ammeters and Voltmeters.—The differ-
erices between ammeters and voltmeters of this type are noted in
|-98. The general principle of both instruments is the same. A
suitably shaped piece of soft iron is drawn into a coil through
which is passed the current to be measured ; or the iron may be
moved from a weaker to a stronger part of the field so produced ;
or the moving iron may be repelled from a stationary piece of
iron magnetised by the same field. In any case the moving iron
is attached to the spindle carrying the pointer, and its movement
takes place \against the control of a spring or gravity (§ 90),
, According to the shape and disposition of the iron, the variation
of deflection with current may be altered within wide limits.*
So long as the iron is unsaturated the flux produced in it varies
with the current in the magnetising coil (§§ 42, 81), hence the
deflecting force on the iron (varying with flux x current) is pro-
portional to the square of the current. The instrument is there-
fore inherently suitable for A.C. measurements (§ 89 &). Due to
hysteresis in the iron (§§ 34,81) there is a tendency for the instru-
ment to read high on descending values. Again, the winding of
a voltmeter of this type is necessarily inductive (§ 98), and unless
the inductance can be " swamped " by plenty of non-inductive re-
sistance (§ 44) the impedance of the instrument circuit, and
therefore the calibration of the instrument, varies with frequency.
It is claimed for the latest moving iron instruments that the error
is less than 1 % whether B.C. or A.C. of 50 or 100 cycles per sac.
be used. In the absence of suitable guarantees, moving-iron
instruments should be calibrated with A.C. of the frequency and
wave form on which they are to be used.
The Dransfield 3-phase voltmeter is a moving-iron instru-
ment with two coils. The latter have one common terminal and
the other ends of the coils are connected through inductance and
* If the scale be logarithmic (like that of a slide rule) overloads of 100 °/0 or so
can be read with sufficient accuracy whilst retaining an open scale within the
normal range.
122